Ruthenium catalysts Noyori-type

Ligand-metal bifunctional catalysis provides an efficient method for the hydrogenation of various unsaturated organic compounds. Shvo-type [83-85] Ru-H/OH and Noyori-type [3-7] Ru-H/NH catalysts have demonstrated bifimctionality with excellent chemo- and enantioselectivities in transfer hydrogenations and hydrogenations of alkenes, aldehydes, ketones, and imines. Based on the isoelectronic analogy of H-Ru-CO and H-Re-NO units, it was anticipated that rhenium nitrosyl-based bifunctional complexes could exhibit catalytic activities comparable to the ruthenium carbonyl ones (Scheme 29) [86]. 

On the other hand, Backvall, Privalov and co-workers proposed an inner-sphere reaction mechanism based on experimental studies and DFT calculations for the reduction of imines with Shvo s catalyst (370,371). The same group demonstrated that imines have to be protonated before being reduced by ruthenium Noyori s catalyst (372). But kinetic studies on the reduction of cyclic imine I (Fig. 94) reported by Blackmond and coworkers (373) conclude that the un-protonated imine is reduced by a Rh(III) catalyst of the Noyori-type catalyst (Fig. 95), using formic acid as the hydrogen transfer agent. 

Recently, Wills and co-workers (344) studied the application of ruthenium catalysts of the Noyori-type catalyst containing W-alkylated TsDPEN derivatives bearing alkyl groups of different size as the chiral ligand for the reduction of cyclic imine I (Fig. 94). The authors suggest that a viable model for the reduction of this imine (Fig. 96) could involve a protonated imine with a transition state allowing the CH-jt interaction between the aryl group of the iminium cation and the arene coordinated to the ruthenium. 

Asymmetric hydrogenation has been achieved with dissolved Wilkinson type catalysts (A. J. Birch, 1976 D. Valentine, Jr., 1978 H.B. Kagan, 1978). The (R)- and (S)-[l,l -binaph-thalene]-2,2 -diylblsCdiphenylphosphine] (= binap ) complexes of ruthenium (A. Miyashita, 1980) and rhodium (A. Miyashita, 1984 R. Noyori, 1987) have been prepared as pure atrop-isomers and used for the stereoselective Noyori hydrogenation of a-(acylamino) acrylic acids and, more significantly, -keto carboxylic esters. In the latter reaction enantiomeric excesses of more than 99% are often achieved (see also M. Nakatsuka, 1990, p. 5586). 

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

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